Enforce metadata string type on key/value pairs

The compute API is currently enforcing quota limits for user metadata
and length restrictions on the dict entries, but is not enforcing that
they are string types. This patch adds the checks for string type on the
keys and values in the metadata dict.

Closes-Bug: #1240319

Change-Id: Ic4aacc342cf37d929871287e6e4917f4f48b63e8
Merge "Enforce metadata string type on key/value pairs"